Early detection of Autism Spectrum Disorders (ASD) has come to be recognized as a crucial activity in promoting the best possible outcome for affected children. In our current NICHD-funded Early Detection (ED) study, for which this application is a competing continuation, we have developed an effective screening instrument, the Modified Checklist for Autism in Toddlers (M-CHAT), which is now widely used in the US, and is being studied in translation in other countries and cultures, including Japan, China, Spain, the Netherlands, France, Iceland, Ireland, Germany, Egypt, Puerto Rico and Turkey. It is being used by the Indian Health Service and is included in the American Academy of Pediatrics screening tool kit. In addition to studying the properties of this screening instrument, studying our current cohort of 16-30 month old children detected with the M-CHAT and diagnosed with ASD, has allowed us to address other questions of great theoretical and practical importance concerning early detection and development, including: characteristics of children with ASD screened from high risk vs. unselected samples, characteristics of younger siblings of children with ASD detected at age 1.5 to 2, early predictors of outcome, the application of DSM-IV symptoms to very young children, diagnostic stability through early childhood, validity of diagnostic instruments in 2-year-olds, growth parameters in early development and their relationships to outcome, and whether general developmental screening is sufficient to detect ASD. The overarching goal of this continuation is to validate a final shortened revision of the M-CHAT. This final version will be (a) brief and user-friendly, (b) easy for pediatricians to score in their offices, (c) valid for diverse populations of children, (d) valid for younger siblings, who are at high risk, and (e) ready to be recommended for universal pediatric use in the US, as well as in multiple translations in other countries. Disseminating a finalized, valid screener for early detection of ASD, which is parent- and pediatrician-friendly, will allow a large number of children to have earlier access to intervention and optimize their developmental outcomes.